1. Field of the Invention
The present invention relates to a gas sensor element which includes a solid electrolyte layer having a plurality of projections on its outer surface, an inner electrode provided on the inner surface of the solid electrolyte layer, an outer electrode provided on the outer surface of the solid electrolyte layer and a protection layer covering the outer electrode for protection of the outer electrode and in which an electrical characteristic value as measured between the electrodes varies according to a component of gas to be detected.
2. Description of Related Art
Conventionally, there is known a structure of and a manufacturing method for an oxygen concentration cell which exhibits such excellent heat resistance that an electrode does not come off even when the cell is exposed to a high-temperature environment for a long period of time. The manufacturing method comprises the steps of: applying a mixture of granules and minute particles onto the surface of a zirconia base body; and firing the base body and the applied mixture simultaneously to thereby form a number of pits and projections on the surface of the base body.
For example, Japanese Patent Application Laid-Open (kokai) No. 56-160653 discloses a method for manufacturing a zirconia oxygen sensor element, including a step of applying a paste of mixed granules and minute particles onto a surface of a zirconia base body at a thickness of 40 xcexcm to 300 xcexcm through use of a brush. According to the publication, projections are formed on the surface of the base body through firing, and the surface is coated with an electrode. As a result, the electrode penetrates into pits formed between projections, thereby increasing its surface area and establishing a strong bond. Also, Japanese Patent Publication No. 2-44391 discloses a method for manufacturing a zirconia oxygen sensor element, including a step of applying a similar paste onto a surface of a zirconia base body in an overflowing manner through use of a roller so as to form a surface layer having a final thickness of 100 xcexcm.
In recent years, an oxygen concentration cell has often been required to be installed downstream of a three-way catalytic converter. Thus, the oxygen concentration cell must function properly in an environment having a temperature of 200xc2x0 C. to 300xc2x0 C. and that is rich with water vapor in which zirconia is apt to undergo phase transition.
However, the oxygen sensor elements described in Japanese Patent Application Laid-Open (kokai) No. 56-160653 and Japanese Patent Publication No. 2-44391 include portions where granules are present in layers, implying the formation of pores. A xe2x80x9cporexe2x80x9d is a minute opening formed between neighboring projections as a result of bonding of tips of the projections or between layered projections. Thus, water vapor enters the pores, causing the phase transition of zirconia to extend to considerably deep portions of projections.
Accordingly, the projections change in volume on a microscopic scale, causing the formation of minute cracks in the projections. Therefore, the projections may break away from the zirconia base body to cause a crack failure.
A gas sensor element includes a solid electrolyte layer having a plurality of projections formed on its outer surface, an inner electrode formed on the inner surface of the solid electrolyte layer, an outer electrode formed on the outer surface of the solid electrolyte layer, an outer electrode formed on the outer surface of the solid electrolyte layer and a spinel protection layer formed on the outer electrode for protection. The projections are formed on the outer surface of the solid electrolyte layer such that they are formed in a single layer; an area occupied by the projections accounts for 10% to 85% of the outer surface of the solid electrolyte layer and the distance between the neighboring projections is 5 xcexcm to 300 xcexcm.
Thus, the outer electrode is formed on the solid electrolyte layer in such a manner as to penetrate into pits formed between neighboring projections and is thus strongly bonded to the solid electrolyte layer, thereby improving thermal durability. Also, since pores are hardly formed or are not formed at all between projections, a strong bond is established among the solid electrolyte layer, the outer electrode, and the protection layer, thereby improving durability against high temperature and high humidity.
In view of the foregoing, an object of the present invention is to provide a gas sensor element exhibiting improved durability against high temperature and high humidity while maintaining a strong bond between a solid electrolyte layer and an outer electrode and a strong bond between the outer electrode and a protection layer, as well as to provide a method for manufacturing the gas sensor element.